what are they?
Butylated Hydroxytoluene- BHT, and Butylated Hydroxyanisole- BHA

what do they do?
These two closely related chemicals are added to oil-containing foods to prevent spoilage in foods that contain oil or shortening. They are also used to preserve breakfast cereals such as Mini-Wheats (whose label the above photo is from). BHA is also used in enriched rice products and dried soups.

what known dangers are there?
The International Agency for Research on Cancer (part of the World Health Organization) considers BHA and BHT carcinogenic. Kate Murphy, in an article entitled "Do Food Additives Subtract From Health?" for Business Week, noted that:

Repeated studies have shown that BHA and BHT increase the risk of cancer as well as accumulate in body tissue, cause liver enlargement, and retard the rate of DNA synthesis and thus, cell development. However, one study, released in 1994, suggests these same additives may actually retard cancer development because of their antioxidant properties.

are they necessary?
"They are totally unnecessary chemical additives. To be safe, read your food labels and avoid foods that contain these potentially harmful chemicals." say the folks at Feel Healthy

history
BHT has been used since 1949 as an antioxidant in many fat-containing foods, in edible fats and oils and in cosmetics.